I don’t think it’s an incorrect use of the semicolon. Semicolons are out of fashion these days, but using a semicolon to join two related clauses is perfectly fine, as far as semicolon usage goes.
I agree that if you think "Wrong root word. You're thinking of Kafir." is acceptable, so too would be the semicolon usage, which is why I said "arguably incorrect" rather than just "incorrect"
"[That is the] wrong root word; You're thinking of Kafir".
Since in the first clause, it seems like the intention is to point to the previous comment that is the matter at hand, which was actually written not just thought about. However, other readings might be possible.